1. Field of the Invention
The present invention relates to hot melt ink formulations, and, in certain embodiments, to hot melt ink formulations in which the ink is light transmissive and can be used, for example, on overhead transparency film, plain paper or backlit displays.
2. The Related Art
Various types of inks have been developed which have been found to be suitable for many types of ink jet printing. Because ink jet devices typically include relatively narrow passages and channels and small orifices through which liquid ink is passed, ink jet inks are generally designed to avoid clogging and to smoothly flow through such narrow passages, channels and orifices. Moreover, ink which has a stable, consistent viscosity is desirable for maximum print quality.
One problem encountered with various prior art inks which contain water or organic solvents is that these inks tend to dry out over time. This can result in dried ink blocking various passages, channels or orifices in the printer. In addition, the loss of volatile solvents can change the viscosity of the ink, which in turn may degrade the performance of the ink and result in poor adhesion to the substrate.
Hot melt inks are typically solid phase at room temperature and liquid phase at the elevated operating temperature of the printer. Examples of such hot melt inks are described in U.S. Pat. No. 3,156,572 to Carlick and Dover. Hot melt inks offer several advantages over conventional liquid inks. First, hot melt inks remain solid during transportation and storage, and will not evaporate like conventional inks do. Thus reliability of the printer is improved because problems associated with nozzle clogging due to evaporation of components of the ink composition are largely eliminated. Furthermore, the ink droplets tend to solidify almost immediately upon contact with the substrate material, which prevents unwanted migration of the ink along the substrate surface and improves the quality of the print.
U.S. Pat. Nos. 4,830,671 and 5,194,638 to Frihart et. al. describes hot melt ink compositions containing a tetra-amide and a colorant. U.S. Pat. No. 4,390,369 to Merritt et al. describes the use of a number of natural waxes, such as Japan wax, candelilla wax, and carnauba wax. These waxes are used as either the ink vehicle or as an additive to other carriers such as fatty acid materials.
None of the above references describe an ink composition or ink carrier for light transmissive inks for use in, for example, overhead transparency applications. Many traditional phase change ink compositions are not particularly light transmissive and thus not effective for creating color transparencies. However, U.S. Pat. No, 4,889,560 to Jaeger et al. describes an ink carrier composition for light transmissive phase change inks, containing both a tetra-amide and a monoamide. Inks made from such compositions, however, have been found to be less light transmissive and durable as desired for many applications.
It would be desirable to have an ink carrier formulation which is highly light transmissive and highly durable. It is to these types of objectives that embodiments of the present ink formulation is directed.